The continuing development of the F-15 has included improvements to its baseline Environment Control System (ECS), an open air cycle system built around a bootstrap air cycle machine. A simple air controller schedule change and the conversion to a High Pressure Water Separator (HPMS) ECS were steps in the evolution of the F-15 ECS which yielded gains in avionics cooling capacity of about 63%. Although there was no associated capacity increase, optimization of the cooling air distribution system was done to improve avionic reliability. Recent modifications of the F-15E aircraft to accommodate the Increased Performance Engines (IPE) have included ECS changes to maintain the capacity gains achieved previously. The higher bleed pressures and temperatures characteristic of the IPE have necessitated new pressure regulators, ducts, and heat exchangers. External scoops have been added to improve ram cooling airflow. Another modification of the F-15E which expands ECS capabilities is a Molecular Sieve Oxygen Generating System (MSOGS) which will be incorporated on all IPE aircraft. This paper discusses the evolution of the F-15 ECS which includes (1) conversion to a HPWS ECS and optimization of the cooling air distribution system and (2) current modifications to accommodate IPE and MSOGS.